The present invention pertains to acquisition of spread-spectrum signals. More particularly, the present invention relates to spread-spectrum signal acquisition in a Global Positioning System (GPS) receiver.
Signal acquisition in a Global Positioning System (GPS) receiver generally requires searching for a modulation code in a carrier signal that is received from a GPS satellite. GPS generally makes use of two spread-spectrum modulation codes known as the xe2x80x9cC/A codexe2x80x9d and the xe2x80x9cP codexe2x80x9d, which are multiplexed in quadrature onto a single carrier. The C/A code is a Gold code with a chipping rate of 1.023 Mbits/sec and is used for making course position determinations. The P code is a pseudo-random code with a chipping rate of 10.23 Mbits/sec and is used for more precise position determinations. The P code is more resistant to jamming than the C/A code and has a secure, xe2x80x9cantispoofxe2x80x9d version known as the Y code. The Y code is available only to authorized users, such as the military. The P code and the Y code are sometimes referred to collectively as the P(Y) code. In GPS receivers as well as other applications, it is desirable to reduce the time needed to acquire a modulation signal, particularly for military applications, in which jamming and other types of interference may be encountered.
Code acquisition in a GPS receiver is generally accomplished by comparing the received code with a reference code generated locally within the receiver in order to synchronize the two codes. The time required to acquire the signal is generally dependent upon frequency and phase uncertainties between the received code and the reference code. Consequently, acquisition of the received code generally involves searching a two-dimensional search region defined by a number of code offset values in one dimension and a number of frequency offset values, sometimes referred to as Doppler offset values, in the other dimension. The code offsets represent different values of phase offset between the received code and the reference code, while the frequency offsets represent different values of frequency offset between the received code and carrier and the reference code and carrier. The code and frequency offset values which define the search region are sometimes referred to as xe2x80x9cbinsxe2x80x9d.
Thus, referring to FIG. 1, the acquisition of a P(Y) code received from a GPS satellite is performed by searching for a signal 20 within a search region 21. The search region 21 is defined in terms of a number of code offset bins along one axis and a number of frequency offset bins along a second axis. Element 22 in FIG. 1 represents a bin. The signal acquisition time is the time from the start of the search until a bin is determined to be occupied or xe2x80x9chitxe2x80x9d.
The process of synchronizing the locally-generated code to the received code in a GPS receiver often involves computing the correlation between the two codes at various points in time. Referring to FIG. 2A, two correlation curves 31 and 32 are associated with two adjacent code offset bins. In the direction of code offset, the shape of the correlation function may be a series of overlapping triangles, the peaks of which are centered at the midpoint of each code offset bin, and the troughs of which fall halfway between each bin. The width of each bin is the width of each triangle at the code offset axis. Although the bins overlap, the midpoints of the bins are spaced apart by C chips; hence, the bins are said to be spaced apart by C chips. The rate at which a search region may be covered generally depends upon the number of parallel search bins and the degree of overlap of adjacent bins. The most adverse condition in terms of signal correlation is for the received signal to be located exactly midway between the center points of adjacent bins. For example, FIG. 2A illustrates that signal correlation is strongest when the actual code offset corresponds to the center of a bin and weakest when the offset corresponds to the trough 33 between two bins. Signal acquisition time, therefore, depends partially upon the xe2x80x9cdepthxe2x80x9d of the trough 33. The depth of the trough 33, in turn, is directly dependent upon the spacing of the bins. If the bins are spaced farther apart, the total number of bins to be searched within a given search region may be reduced; however, the overall acquisition time may increase due to the increased trough depth. These principles, therefore, give rise to a design trade-off between bin spacing and signal acquisition speed.
FIG. 2B shows a relationship between frequency offset and signal correlation. In the direction of frequency offset, the shape of the correlation function is a series of overlapping sinc(x) curves, the peaks of which are centered at the midpoint of each frequency offset bin and the troughs of which fall halfway between each bin. Curves 34 and 35 represent the correlation curves associated with two adjacent frequency offset bins. The bins associated with curves 34 and 35 overlap but are spaced apart by F Hz. The width of the frequency offset bins is the width between the nulls of each sinc(x) curve at the frequency offset axis. As with code offset, the depth of the trough 36 depends partially upon the spacing of the bins.
GPS receivers conventionally use Fast Fourier Transform (FFT) techniques for signal acquisition. One problem associated with FFT-based receivers, however, is that the spacings of code offset bins and frequency offset bins are fixed. In conventional parallel search devices, the spacing of successive code offset bins is determined by the sampling rate of the analog to digital converter. Similarly, the spacing of frequency offset bins is determined by FFT-derived frequencies, which, in turn, are a function of the analog-to-digital sampling rate. Consequently, in conventional GPS receivers which use FFTs for code acquisition, the spacing between bins may be optimal for a given device and signal environment, yet poor for other devices and signal environments.
Another problem generally associated with code acquisition in GPS receivers is loss due to xe2x80x9ccode smearingxe2x80x9d, i.e., losses which result when the received code is not synchronized with the reference code over the entire correlation interval due to code Doppler. Code smearing tends to be worse in the outer frequency offset bins and often results from a total frequency offset that is too wide, resulting in divergence of the code and carrier frequencies. Because of their fixed bin spacing, FFT-based receivers tend to be highly susceptible to code smearing losses.
Hence, it is generally desirable to reduce signal acquisition time and losses when acquiring a spread-spectrum signal in a changeable signal environment. More specifically, it is desirable to reduce acquisition time and code smearing losses during acquisition of modulation codes in a GPS receiver under variable signal conditions.
One aspect of the present invention is a method of searching for a signal within a search region defined in terms of a number of bins. The method includes varying the spacing of the bins and attempting to locate the signal in one of the bins. In specific embodiments, varying the spacing of the bins includes dynamically varying the spacing of the bins based on a current signal environment.
Another aspect of the present invention is a receiver for searching for a signal within a search region. The search region is defined in terms of a number of bins. The receiver includes an input stage for receiving and sampling an input signal. An accumulation stage is coupled to the input stage. The accumulation stage combines at least some of the samples based on a first control input, such that the spacing of the bins is variable based on the first control input.
Other features of the present invention will be apparent from the accompanying drawings and from the detailed description which follows.